Heat resistant heat shrinkable film

ABSTRACT

A three layer heat shrinkable film is disclosed, along with preferred process for making it. The film comprises a first heat sealable layer of an ionomer, a second layer of a vinylidene chloride copolymer, and a third layer of a blend of an ionomer and an EVA copolymer. The EVA content is no greater than 20% of the blend by weight, and the vinyl acetate content is at least 4.5% of the blend by weight. In the preferred process, a three layer tape is coextruded and cooled, and reheated to 197°-203° F., and oriented in a conventional double bubble process.

BACKGROUND OF THE INVENTION

Heat shrinkable polymer films have gained substantial acceptance forsuch uses as the packaging of foodstuffs. This invention pertains toimprovements in heat shrinkable films of this nature. The filmsembodying this invention are normally used as heat shrinkable bagssupplied to the food processor with one open end, to be closed andsealed after insertion of the product. After the product is inserted,air is normally evacuated, the open end of the bag is closed, such as byheat sealing, or other means, and finally heat is applied, such as byhot water, to initiate film shrinkage about the product.

Successful shrink bags must satisfy a multiplicity of requirementsimposed by both the bag producer and the bag user. Of primary importanceto the bag user is the capability of the bag to survive physicallyintact the processes of being filled, evacuated, sealed closed and heatshrunk. The bag must also be strong enough to survive the materialhandling involved in moving the contained product along the distributionsystem.

The bag producer desires a product which can be produced competitivelyon conventionally available equipment, such as shown in U.S. Pat. No.4,161,562, while meeting the performance requirements of the user. Thusthe bag material should be readily extrudable and susceptible toorientation with sufficient leeway in process parameters as to allow forefficient film production. The orientation temperature should be atemperature which is economically achieved by the producer, and whichprovides for use of economical shrink processes by the bag user.

Conventional shrink bags have generally been constructed of layerscomprising predominantly ethylene vinyl acetate (EVA) copolymers whichmay or may not be crosslinked, and, in some cases, contain a layer ofvinylidene chloride copolymer. However, the use of structures containingpredominantly EVA may not always be desirable.

As will be appreciated, the processes of stretching the film, and latershrinking it, expose the film to rather severe physical stresses, due tothe nature of the operations. In between the stretching and shrinkingoperations, the film may be subjected to rather harsh tests of itsphysical capabilities by the various handlings and environments to whichit is exposed. It may, for example be shipped in a closed truck exposedto the sun's heat. Thus initiation of the shrink properties must not betriggered at too low a temperature.

In some cases it is desirable to have a shrink bag having lower shrinkforce than is conventionally experienced without sacrificing the amountof shrink. For example, in packaging blocks of cheese, excessive shrinkforce yields rounded corners on the soft product. Simultaneously, it isimportant to retain the conventional amount of shrinkage to ensure filmconformity to the product.

In the production of bags, the bags are frequently mounted on adhesivelytaped rolls; wherein the user removes a bag by forcefully pulling it offthe adhesive tape. Thus the interlayer adhesion in the multiple layerbag structure must be sufficient to ensure that the layers of the bagremain adhered to each other during the removal process, helping toprovide for the overall internal cohesion of the structure. The bag musthave a balance of heat sealability properties, such that the bag can beheat sealed closed over a range of conditions acceptable to commercialoperation without excessive faulty seals or burn-throughs. Finally, allthese properties must be met in a bag which can be easily laid flat in ashipping container, such as a cardboard box, without undue curling ortwisting of the bags.

It is an object of this invention to provide a novel heat shrinkablefilm which gives improved control of the shrink force while maintaininga high degree of shrink capability.

It is another object to provide such a film which has good heatsealability characteristics and structural cohesion.

It is still another object to provide a process for making a multiplelayer tubular film which provides improved control of the shrink processand has good heat sealability characteristics.

SUMMARY OF THE INVENTION

It has now been found that certain improvements in heat shrinkable filmare achieved in three layer films wherein the first layer is a heatsealable ionomer, the second layer is a vinylidene chloride copolymer,and the third layer is a blend of an ionomer and an EVA copolymer. TheEVA content of the blend is no greater than 20% of the blend by weight,and the vinyl acetate content is at least 4.5% of the blend by weight.In preferred embodiments, the EVA content is between about 15% and 20%of the blend by weight and the vinyl acetate content is about 22% to 33%of the EVA copolymer by weight.

The invention is further exemplified in a heat shrinkable bag made fromthe three layer film disclosed, wherein the heat sealable ionomer layeris disposed on the inside surface of the bag, and is heat sealed atfacing portions of itself when the ends of the bag are closed. The blendlayer is on the outside surface of the bag and contacts the heating sealjaws during the heat sealing operation.

The invention is also exemplified in a process for making a three layerheat shrinkable film. The process is initiated by coextruding a threelayer tape having first, second and third layers as hereinbeforedisclosed. The tape is cooled below the crystalline melting temperatureof each of the three layers, and then reheated to a temperature of about197°-203° F., and oriented to form a film by blowing a bubble in theheated tape and providing an appropriate draw to the film to provide anoriented film having normally balanced shrink properties.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

After much work and exploration, the inventors herein have found athree-layer structure which satisfies the requirements of both the bagproducer and the bag user.

The inside layer of the bag is heat sealable ionomer. The middle layeris vinylidene chloride copolymer. The outer layer is a blend of ionomerwith an EVA copolymer, such that the vinyl acetate content of the entireblend is at least about 4.5% by weight of the blend, while the EVAcontent is no more than about 20% by weight of the blend. In meeting thecriteria of at least 4.5% vinyl acetate without exceeding 20% EVA, it isseen that the vinyl acetate content of the EVA must equal at least 22%.Preferrably, the vinyl acetate content equals at least 5% of the blend;thus the normal range of vinyl acetate content of the EVA used in thisinvention is 25% to 33% vinyl acetate, the 33% material being thehighest vinyl acetate content available for coextrusion. It isanticipated that, as higher vinyl acetate content copolymers becomeavailable, these, too, will be found advantageous for use with theinvention. At present, with 33% vinyl acetate being the highestconcentration available, a minimum of 15% EVA, based on blend weight, isrequired. Thus, the EVA is desirably present as 15% to 20% of the blend,with the copolymer being desirably 25%-33% vinyl acetate.

The minimum requirement of 4.5% vinyl acetate is directly related to thegeneration of adhesion between the outer blend layer and the middlelayer of vinylidene chloride copolymer, the 4.5% vinyl acetate beingadequate to raise the adhesion level to about 40 grams per inch width,compared to about 4 grams per inch width for pure ionomer in the outerlayer.

The maximum of 20% EVA is determined by the needs of the structure.Primarily, a maximum amount of ionomer is desired to enhance heatsealing properties by providing good heat resistance during the heatsealing operation, so that strong seals may be formed without burningholes in the film adjacent the heat seal areas. Secondly, in higheramounts, the EVA is incompatible with ionomer, yielding a cloudy film.Finally, having major proportions of ionomer in both the inner and outerlayers is effective to balance the film so that it naturally lays flat,and does not curl, as do some films which have high proportions ofdifferent polymers on their opposing faces.

Films of this invention are conveniently made on equipment common to the"double bubble" process. In this process, the film is extruded first asa thick-walled tubular tape, cooled below the crystalline meltingtemperature of each of the three layers, then reheated and oriented as abubble to form the film of the invention. Finally, the film is cooled,the bubble is collapsed, and wound up. While the film may be made over asubstantial range of operating conditions, it is desirable for thereheat temperature to be at least 195° F. and no more than 205° F. Amost preferred temperature range for reheating the tape is 197° to 203°F.

EXAMPLE 1

A tubular three layer tape having sidewalls 16 mils thick was preparedby coextrusion from a 4.5 inch circular die at 320° F. The width of thecollapsed tape was 5.85 inches. The outer layer of the tape was about2.8 mils of a blend of 20% by weight of DuPont Elvax 360 EVA having 25%vinyl acetate content, and 80% by weight DuPont Surlyn 1601 ionomer. Thesecond layer was about 2.8 mils of Dow Saran vinylidene chloridecopolymer. The third layer was about 10.4 mils of DuPont Surlyn 1601ionomer. The tape was cooled to 77° F., then reheated to 197° F. andbiaxially oriented in a conventional bubble blowing process. The blow-upratio was 3.1. The bubble was stretched in the machine direction at aratio of about 2.3 by having the nip rollers downstream of the bubblepull the tube at a rate 2.3 times as fast as the nip rollers upstream ofthe bubble let the tape pass. The bubble was finally collapsed and woundup.

The resulting film was 2.25 mils thick; the heat sealable inner layer ofSurlyn being 1.45 mils, the Saran layer being 0.4 mil, and the outerblend layer being 0.4 mil.

EXAMPLE 2

A biaxially oriented film was produced as in EXAMPLE 1 except that thereheat temperature was 203° F.

EXAMPLE 3

A biaxially oriented film was produced as in EXAMPLE 1 except that 80%Surlyn 1652 ionomer was used in the third layer in place of the 80%Surlyn 1601.

EXAMPLE 4

A biaxially oriented film was produced as in EXAMPLE 3 except that thereheat temperature was 203° F.

The films from the examples were tested for free shrink and shrinkforce, and compared to similar three-layer EVA-based films, both across-linked film and an uncross-linked film. Shrink force was testedaccording to ASTM D-2838. In the percent free shrink test, squaresamples were cut 100 millimeters on a side and marked for identificationin the with machine direction and the cross machine direction. Eachsample was placed between two screens and immersed in hot water at thespecified temperature for 60 seconds. The samples were withdrawn fromthe water, dried, and measured in both the with machine and crossmachine directions. The amount of shrinkage, in millimeters was noteddirectly as the percent free shrink.

The results are shown in Tables 1 and 2, with each result being thegeometric mean of the readings taken in the with machine directions andthe cross machine direction.

                  TABLE 1                                                         ______________________________________                                        Percent Free Shrink                                                                          Shrink Temperature                                             Film             160° F.                                                                         180° F.                                      ______________________________________                                        EVA              10%      25%                                                 EVA crosslinked  10%      25%                                                 Example 1        26%      45%                                                 Example 2        21%      43%                                                 Example 3        24%      45%                                                 Example 4        20%      42%                                                 ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        Shrink Force, grams/inch                                                                     Shrink Temperature                                             Film             160° F.                                                                         180° F.                                      ______________________________________                                        EVA              213      281                                                 EVA crosslinked  274      273                                                 Example 1        219      188                                                 Example 2        179      158                                                 Example 3        186      171                                                 Example 4        160      160                                                 ______________________________________                                    

As evidenced by the data in Table 1, the films of this invention have asubstantially higher percentage of free shrink than either of the EVAmaterials, compared at either shrink temperature. Further, as seen inTable 2, the shrink force in all the examples is more moderate at the180° temperature than either of the EVA-films. At 160° theuncross-linked EVA has a mild shrink force similar to that of Example 1,but is substantially poorer in the amount of free shrink shown inTable 1. Thus the data bears out the superiority of the films of thisinvention in maximizing shrink amount while moderating the shrink force.

The bag producer must be particularly concerned with the ability of thebag to survive all of the stresses to which it may be exposed during itsfunctional life. While adhesion of the vinylidene chloride copolymer tothe heat seal layer is not particularly of concern, the adhesion of thevinylidene chloride copolymer to the outer layer is very important inorder to ensure that the outer layer is not torn off the bag when thebag is removed from the mounting tape. Thus a pure layer of ionomer asthe outer layer is not safisfactory. However, a high amount of ionomeris desirable, to provide good heat resistance during the heat sealingoperation, particularly in forming the final seal after the product isinserted into the bag. It is thus critical that the outer layer have ahigh percentage of ionomer, while at the same time, that layer must havean adequate adhesion to the vinylidene chloride copolymer. A furtherbenefit of having major proportion of ionomer in both the inner andouter layers is that it gives effective balance to the film and thusgives it desirable lay-flat characteristics.

Film made by this process may be used with conventional shrink tunneltemperatures on the order of 200° F., but, as shown by the data, mayalso be used with temperatures as low as no more than 160° F. Thus thesefilms are adaptable to commercial operating conditions found on asubstantial variety of commercial equipments and processes, includingboth the older processes operating at about 200° F., and the newerprocesses operating at about 160° F. to 170° F.

The films made in the Examples were made into bags on conventional bagmaking equipment, by sealing one end of a section of the tubular film.The bags were then filled with cheese, heat sealed on conventionalcommercial equipment at normal operating speeds, and observed and testedfor leakers. As no leakers were found in the sealed packages, good sealintegrity and heat seal capability was affirmed.

The invention is of particular utility, especially in the packaging ofcheese, a soft product, wherein it is important to have the low shrinkforce of the films of the invention, in combination with a highpercentage of shrink amounts. While the inventors are unable to explainthe behavior of the films of this invention, low shrink force ispossibly a result of the blend ratios used and the low reheattemperature.

Thus the invention provides novel heat shrinkable films which giveimproved control of the shrink force while maintaining a relatively highamount of free shrink, even at low shrink temperatures. Further the filmhas good heat sealability characteristics and structural cohesion.

Finally, the invention provides a novel process for making the film,wherein it is believed the process is at least partially responsible forthe resultant beneficial properties of the film.

Having thus described the invention, what is claimed is:
 1. A heatshrinkable bag made from a three layer tubular film, the filmcomprising:(a) a first heat sealable layer of ionomer; (b) a secondlayer of a vinylidene chloride copolymer; and (c) a third layer of ablend of an ionomer and an EVA copolymer, wherein the EVA content is nogreater than 20% of the blend by weight, and the vinyl acetate contentis at least 4.5% of the blend by weight.
 2. A heat shrinkable bag as inclaim 1, wherein the EVA content of the film is between 15% and 20% ofthe blend by weight and the vinyl acetate content is 22% to 33% of theEVA copolymer by weight.
 3. A heat shrinkable oriented three layer film,comprising:(a) a first heat sealable layer of an ionomer; (b) a secondlayer of a vinylidene chloride copolymer; and (c) a third layer of ablend of an ionomer and an EVA copolymer, wherein the EVA content is nogreater than 20% of the blend by weight, and the vinyl acetate contentis at least 4.5% of the blend by weight.
 4. A film as in claim 3 whereinthe EVA content is between about 15% and 20% of the blend by weight andthe vinyl acetate content is 22% to 33% of the EVA copolymer by weight.